1. Field of Application
The present invention relates to a circuit for eliminating afterimages in a picture produced by a liquid crystal display apparatus, which occur as a result of the transient response characteristics of the liquid crystal material.
2. Prior Art Technology
In recent years there has been an increasing trend towards making the display devices of various types of electronic apparatus increasingly thin and compact. For that reason, liquid crystal display devices have come into widespread application in television receivers, personal computers, etc. Such liquid crystal display devices are generally of reflected-light or backlit types, however in some cases liquid crystal display elements are used as light valves for modulation of light beams used in a projection type of display. Liquid crystal display elements are formed of a thermotropic liquid crystal material, with the operation generally being based upon changes in molecular alignment of liquid crystal which is in the nematic phase, in response to changes in an electric field that is applied to the liquid crystal. In particular, the twisted-nematic mode of operation is now widely utilized, in which liquid crystal material having a twisted-nematic molecular arrangement is enclosed as a thin layer between a pair of optical polarizer plates. With such a display device, in the absence of an applied electric field, the liquid crystal molecules are oriented such as to rotate the plane of polarization of light which passes through the liquid crystal layer. If the polarizer plates are mutually aligned such that light is enabled to pass through the combination of the polarizing plates and liquid crystal layer in that condition, then the display operates in a mode generally referred to as the normally open mode. When an electric field that is higher than a certain threshold voltage is applied to the layer, i.e. is applied to a display element, the liquid crystal molecules become aligned such that polarization by the liquid crystal layer ceases. As a result, with the normally open mode, the light is now blocked from passing through the display element, i.e. the dark, or closed state occurs. If the polarizing plates are mutually aligned such that light is blocked from passing through the combination of the polarizing plates and liquid crystal layer when no electric field is applied to the liquid crystal, then that display mode is generally referred to as the normally closed mode. In that case, application of an electric field to a display element will result in the light (i.e. open) state being produced.
A liquid crystal display device has a basic disadvantage, by comparison with other types of display device such as a CRT for example, in that the transient response of the liquid crystal to changes in an applied electric field is poor. That is to say, due to the viscosity of the liquid crystal material, there is a delay between the point in time at which the electric field is changed and the time at which a resultant change in the molecular alignment (and hence, a corresponding change towards the light or dark display condition) is completed. In the following, the time required for the liquid crystal material to complete a change in state in response to an electric field being applied (starting from an initial condition of zero applied electric field) will be designated as the rise time t.sub.r of the liquid crystal, and time required for the liquid crystal to complete a change in state in response to an electric field being removed (i.e. starting from an initial condition of that electric field being applied) will be designated as the fall time t.sub.d of the liquid crystal. When liquid crystal display elements are used to display a static image, the poor transient response characteristic does not present a problem. However when a moving image is to be displayed, the relatively high values of rise time and fall time result in afterimages being produced on the display. This seriously degrades the quality of the picture obtained with an liquid crystal display apparatus, and is a basic disadvantage of such a display apparatus.
This is a particular problem when liquid crystal display elements are to be used to implement a 3-dimensional television display of the sequential-screen type. Due to the poor transient response characteristic, there will be spatial deviations between the images that are to be successively presented to the left and right eyes of the viewer, i.e. images which occur successively along the time axis. Another example of the disadvantages of the poor transient response characteristic occurs in the case of television game displays, in which very rapid changes in image position can occur, so that the afterimages represent a substantial problem.
The aforementioned rise time t.sub.r and fall time t.sub.d of the liquid crystal can be formally expressed as follows: EQU t.sub.r =.eta.d.sup.2 (.DELTA..epsilon.V.sup.2 -Kii.pi.(.sup.2).sup.-1( 1) EQU t.sub.d =.eta.d.sup.2 /Kii.pi..sup.2 ( 2)
In the above, V is the applied voltage (i.e. a transient changes between applied voltage values of zero and V occur). .DELTA..epsilon. denotes the inductive anistropy of the liquid crystal, Kii is the elastic coefficient of the liquid crystal, .eta. is the viscosity of the liquid crystal, and d is the thickness of the liquid crystal layer.
The rise time t.sub.r of the liquid crystal determines the time taken for a change in a display element from the light state to the dark state in response to an increase in the voltage applied to the display element, in the case of a liquid crystal display apparatus which operates in the normally open mode. However in the case of a liquid crystal display apparatus which operates in the normally closed mode, the rise time t.sub.r determines the time taken for a change from the dark to the light state in response to an increase in the applied voltage level. Similarly, the fall time t.sub.d of the liquid crystal determines the time taken for a change in a display element from the dark state to the light state in response to a decrease in the applied voltage, in the case of a liquid crystal display apparatus which operates in the normally open mode. However in the case of a liquid crystal display apparatus which operates in the normally closed mode, the fall time t.sub.d determines the time required for a change in a display element from the light to the dark state in response to a decrease in the applied voltage.
The above equations approximately express the relationships between applied voltage and the rise time and fall time of a liquid crystal display element. However it is now known in the art that although the equation (2) indicates that the fall time of a liquid crystal display element is independent of the amplitude of applied voltage, the fall time actually does in fact depend upon the applied voltage. FIG. 1 illustrates the actual relationships between the rise time t.sub.r and fall time t.sub.d of a liquid crystal display element and the amplitude of voltage applied to the display element. It can be seen that a substantial decrease in the rise time t.sub.r will be achieved by increasing the applied voltage, and that a decrease in the fall time t.sub.d can also be achieved by increasing the applied voltage, although the degree of decrease of the fall time with increase of the applied voltage is smaller than for the rise time t.sub.r.
Thus, the relationship between t.sub.r and the level of applied electric field, i.e. the video signal level, is different from the relationship between t.sub.d and the level of applied electric field. It is therefore not possible to achieve satisfactory elimination of afterimages unless signal are derived for use in that elimination which are respectively appropriate for the cases of afterimages resulting from the rise time and afterimages resulting from the fall time of the liquid crystal. That has not been achieved in the prior art.